Feeling totally wiped out

Am just getting over a viral infection which has made a condition that I have called costochondritis flare up. I went back to work on Thursday after a four day break but feel incredibly fatigued. Cooking dinner tonight has wiped me out. I haven't been able to exercise like I normally would and this tends to make me achey anyway. I was wondering whether anyone has any tips? I feel like a need a "tonic" like they used to say years ago but I'm not convinced anything like that which you can buy yourself in a supermarket is really that potent to help much.